The Systemic Control of Tomato Blight

Blight Identification

• Septoria leaf spot appears as small, round, water-soaked spots about 1/8 inch in diameter in wet, warm weather. These develop grayish-white centers. The lower leaves turn yellow and drop. If the disease persists, the higher leaves turn yellow. Early blight appears as dark-brown-to-black spots on the lower leaves late in the season after a heavy set of tomatoes. Yellow spreads from rings that develop around the spots. Yellow leaves eventually drop. The fungus causes cankers on the stems and black spots on the stem end of the tomatoes, causing them to drop prematurely. Late blight appears during cool, rainy weather or dew following cool, night temperatures. The fungus forms irregular, greenish-black blotches that give make the plant look like it was frost damaged. The leaves' bottoms look downy-white. Irregularly shaped blotches appear on both green and ripe tomatoes.

Cultural Controls

• Use the same growing practices for controlling all three fungi. Plant seeds from disease-free plants. When possible, use seeds from tomato varieties that resist or tolerate blight fungi. Plant tomatoes wide enough for good air circulation and in rows aimed at prevailing winds. Do not plant them in the shade or next to wind barriers. Avoid splashing water on your plants. Do not water tomato plants with sprinklers in the late afternoon or night. Irrigate them in the morning so the foliage rapidly dries. Remove and destroy the plants after harvest. If this is not possible, plow the plants under. "Phytophthora: in Phytophthora infestans is Latin for "plant destroyer." When late blight becomes severe, remove and destroy infected plants immediately. Rotate tomatoes with nonsusceptible crops every 3 years.

Fungicides

• Horticulturalists at the University of Maine recommend applying fungicides containing the active ingredients azoxystrobin, chlorothalonil, mancozeb, maneb, ziram or copper-based products for seven to 14 days following early blight's first symptoms. Apply the biological control Bacillus subtilis five to seven days after symptoms appear or potassium bicarbonate five to 14 days as required. Mississippi State horticulturalists report these fungicides also treat septoria leaf spot. University of Massachusetts horticulturalists report there are no fungicides that eliminate late blight after infection. Apply preventative or protective fungicides, including those that are copper based or contain the active ingredient chlorothaloril or mancozeb.gredients chlorothaloril or mancozeb.

Disease Resistant Varieties

• The tomato cultivars Mountain Magic and Plum Regal are resistant to all three fungi. Cornell University horticulturalists say tomato cultivars Allegany, Defender, Elba, Jaqueline Lee, Kennebec and Ozette all resist late blight with Elba being the most resistant. Matt's Wild Cherry and Sun Gold Cherry are cherry tomatoes that resist late blight.